Currently, the popularity of mobile communication devices such as cellular telephones, including smartphones and so called superphones, continues to increase as more and more people around the world adopt them for both personal and business communications. Such mobile communication devices enable users to place and receive phone calls almost anywhere they travel. As the technology incorporated within mobile communication devices advances, so too has the functionality of these devices. Many mobile communication devices provide numerous advanced features in addition to the basic telephone calling ability. Such features include for example, wireless Internet browsing via a cellular data network (e.g., 3G, 4G) or available wireless local area network (WLAN) hotspot, wireless email, calendaring, address book, task lists, calculators, word processing, spreadsheets, etc. In addition, the more advanced mobile communication devices have the capability of running applications (referred to as ‘apps’) that provide specific functionality to the device. Such applications (typically free or low cost) are downloaded from the Internet and installed on the device.
The increase in the functionality of mobile communication devices is also driving the demand for smaller and smaller devices that are easier and more convenient for users to carry. This is putting pressure on mobile communication device designers to shrink the physical size of internal circuit boards and electrical/electronic components within the device. This causes many components to be located closer together, especially the radio frequency (RF) components such as the antenna, microphone components, RF power amplifiers, etc. This increases the possibility that the various electronic components in the device will suffer from electromagnetic interference (EMI) either from RF components and subsystems in the device and/or from external sources. For example, an internal surface mounted microphone could pick up conducted energy directly from an RF power amplifier or from the energy radiated by the antenna. This unwanted reception of conducted/near field radiated energy from power amplifiers and antennae may be particularly problematic in burst transmission schemes such as a Global System for Mobile communications (GSM) system.
Sources of other interfering EMI signals in some mobile wireless communications devices include the liquid crystal display (LCD), microprocessor or central processing unit (CPU), clock generator circuits, etc. which radiate RF energy, possibly interfering with unshielded components thereby degrading device performance. Additional problems may occur when the conducted and radiated interfering RF energy is coupled to the mobile communications device causing audio break through tests to fail for both the uplink and downlink. Even keyboard circuits can potentially create unwanted EMI problems. For example, RF receiver sensitivity is often degraded by the EMI of spectral harmonics emitted from the microprocessor or CPU via the keyboard because of the resulting loop formed by any keyboard circuits. In some instances, strong RF energy, for example, the transmitted power from the radio via the antenna interferes with or couples to the microprocessor or CPU input/output (I/O) lines of the mobile communications device through the keyboard Key-In and Key-Out lines and causes a reset of the microprocessor or CPU.